Hereinafter, explanation will be made by supplying a definition of the “Internet” for the network that connects mutually neighboring routers of respective terminals. More concretely, when data are transmitted from the terminal in a house to the terminal in another house, the sent data are transmitted from the in-house router via the access line such as ADSL, optical line, or the like, the lines of a plurality of ISPs (Internet Service Providers), and the like. In addition, the sent data are delivered to the terminal in another house via the access line and the in-house router connected to another house. The network laid from this in-house router to another in-house router is called the Internet.
It is difficult to ensure a fixed bandwidth between any terminals connected to the Internet. In the prior art, as the system of reserving the band between the routers in the Internet, INTSERV (Integrated Services) used to ensure the bandwidth by using RSVP (Resource ReSerVation Protocol) every communication between respective terminals is known. However, this INTSERV has not been put to practical use because the same system must be applied to all routers that plural ISPs manage on the Internet. Therefore, the communications handled by all terminals share the bandwidth among them on the Internet, i.e., the communication is carried out on a so-called best effect basis.
As the communication system used to share fairly the bandwidth among the communications handled by respective terminals on the Internet that performs a best effort type service, TCP is widely used. This TCP is used in the electronic mail, the P2P (peer to peer) file exchange communication such as WWW (World Wide Web), Winny, or WinMX, and the like, and takes up most of the Internet traffic. Therefore, in the communication on the Internet that executes the best effort type service, it is recommended that the congestion control should be accomplished by controlling the transmission rate to share fairly the bandwidth with this TCP (Reference Literature: “Congestion Control Principles”, RFC 2914).
Meanwhile, TCP exhibits the characteristic called AIMD (Additive Increase Multiple Decrease) in the transmission rate control. Since the transmission rate is changed drastically due to this characteristic, this TCP is unsuited to the streaming that transmits the continuous information such as sound, moving picture, and the like via the network.
In order to overcome this problem, TFRC that can share fairly the bandwidth with TCP while achieving the smooth transmission rate control has been developed.
This TFRC is the method that estimates a transmission rate (X), at which TFRC can share fairly the bandwidth with TCP, based on a loss event rate (p) and a round trip time (R) described later to control the transmission rate. Concretely, the transmission rate (X) is estimated by Eq. (1) given as follows.
                    [                  Formula          ⁢                                          ⁢          1                ]                                                                                      ⁢                  Xcal          =                                    8              ⁢              s                                      R              ⁡                              (                                                                            2                      ⁢                                              p                        /                        3                                                                              +                                      t_RTO                    ×                                                                  3                        ⁢                                                  p                          /                          8                                                                                      ×                                          (                                              1                        +                                                  32                          ⁢                                                      p                            2                                                                                              )                                                                      )                                                                        (        1        )            
In the transmission rate control using TFRC, the estimation of the transmission rate using this Eq. (1) is executed every time when a loss occurs (see FIG. 1).
In order to achieve smooth transmission rate control in the transmission rate control of this TFRC, the loss event rate (p) is calculated by taking account the loss information collected several times (eight times in FIG. 1) in the past.
In TFRC, both a quick congestion avoidance, which should be taken when the Internet is brought into an excessive congestion state, and the smooth rate control can be attained by executing such control. That is, a number of loss events occurs in a short time when the congestion state occurs on the Internet. Thus, the loss event rate observed by the receiving terminal has instantly a large value. Because this value is reported to the sender every loss, the transmission rate can be calculated immediately by Eq. (1) and is applied, an amount of flow of calls can be suppressed. As a result, this operation acts as congestion avoidance on the Internet. Conversely, when an excessive congestion state does not occur on the Internet and the loss does not occur so much, a frequency of occurrence of the loss is lowered and also the loss event rate observed on the receiver side is reduced. As described above, since the loss event rate is calculated by taking account a loss history, the loss event rate is decreased smoothly in time and accordingly the transmission rate is increased smoothly. In summary, a quick congestion avoiding operation is taken when the congestion occurs on the Internet, while the transmission rate is varied (increased) smoothly when the communications recover from the congestion state.
Also, in TFRC, an operation of increasing the transmission rate gradually from a transmission rate=1 packet/round trip time (called a slow start) is taken in starting the transmission, like TCP. However, because the transmission rate is varied smoothly in TFRC rather than TCP, it takes much time until the transmission rate reaches a rate that is required to send the streaming. Also, in the streaming that regenerates the data while receiving the data continuously, the data cannot be played on the receiver side unless the transmission rate that is in excess of a playing rate can be maintained. Therefore, the receiving terminal needs much time to start actually the regeneration of video, etc. after such terminal started the reception of the streaming.
For this reason, when the user switches the receiving terminal from the particular terminal to a new terminal, a lot of queuing time is required of the user.
Meanwhile, such a system has been proposed that, when the streaming is to be transmitted to a new receiving terminal, the other terminal informs the concerned receiving terminal at what extent of the transmission rate the streaming can be transmitted (see Patent Literature 1, for example).
In this system, when the new terminal is connected to the Internet and then is logged in a particular server (e.g., server for exchanging instant messages) on the Internet, a bandwidth measuring server on the Internet measures an effective bandwidth between the new terminal and the bandwidth measuring server. Then, the bandwidth measuring server informs another terminal of the information about the measured effective bandwidth via the particular server. Then, the new terminal holds the communication with another terminal by using this information.
With this system, the transmission rate that is adequate to the streaming executed between the new terminal and another terminal can be decided roughly by using a value that is close to the effective bandwidth of the new terminal. Concretely, when the effective bandwidth between the new terminal and the bandwidth measuring server is 1.5 Mbps, the user gives up the idea of transmitting the high-definition moving picture whose maximum transmission rate is 6 Mbps, or the like. Then, instead of this moving picture, the user can transmit the low-definition moving picture whose maximum transmission rate is almost 1 Mbps.    Patent Literature 1: JP-A-2004-129205